1. Field of the Invention
This invention relates to optical beam mixing methods and apparatus, and more particularly to spatial beam mixing such as might be required in stimulated Brillouin scattering (SBS) phase conjugation with an amplifier-oscillator configuration, in which a seed beam is amplified by a pump beam having a non-uniform spatial intensity distribution.
2. Description of the Related Art
Phase conjugation is an optical phenomenon that has attracted considerable attention in recent years. Several different ways of producing phase conjugated beams have been discussed in the literature, including four-wave mixing, stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), three-wave mixing and photon echo devices. A review of various applications for optical phase conjugation is presented by Giuliano in Physics Today, "Applications of Optical Phase Conjugation", April 1981, pages 27-35. A general review of the field is given in A. Yariv, "Phase Conjugate Optics and Real-Time Holography", IEEE J. Quantum Electronics, Vol. QE14, No. 9, pages 650-660 (1978), and in The Laser Handbook Vol. 4, edited by M. L. Stitch and M. Boss, chapter 4, by D. Pepper "Non-Linear Optical Phase Conjugation", pages 333-485, North Holland Publishing Co. 1985.
A phase conjugate mirror (PCM) produces a retro-reflection of an incident beam, with the phase of the reflected beam reversed from that of the incident beam at the point of reflection. PCMs can be provided either with external pump beams, as in the four-wave mixer, or as a "self-pumped" device which eliminates the requirement for external pump beams. Of the self-pumped PCMs, those employing SBS or Raman scattering are generally used in connection with high power pulsed laser beams, such as from a Nd:YAG laser.
When any material is penetrated by light of an intensity great enough the material is modified, as is also the light which penetrates it. In SBS, the modified material generates sound waves that reflect phase conjugated light waves. An introduction to practical applications of optical phase conjugation, including detailed presentations of SBS and four-wave mixing, is presented in D. Pepper, "Applications of Optical Phase Conjugation", Scientific American, January 1986, pages 74-83.
SRS is described in, among other places, N. Bloembergen, "The Stimulated Raman Effect", American Journal of Physics, Vol. 35, November 1967, pages 989-1023. The Raman effect may be described as the scattering of light from matter, such as a gas, liquid or solid, with a shift in wavelength from that of the usually monochromatic incident radiation. The internal degrees of freedom (for example, electronic, vibrational or rotational) of atoms or molecules of the medium couple the incident radiation such that spatial variations within the medium result in a scattering of the incident radiation. This follows from the fact that the optical properties of the molecules vary with the excitation of their internal degrees of freedom.
One type of PCM that is beneficial for lasers in the medium and high energy range (more than five joules/pulse) employs amplifier and oscillator sections that exhibit SBS. Such a system is illustrated in FIG. 1. The overall PCM 2 is shown enclosed within a dashed line. It includes an amplifier section 4 and an oscillator section 6 that are formed from the same SBS medium, or at least from SBS media that have equal sound velocities. This is necessary to tune the amplifier to the same frequency as the oscillator and thereby avoid degradation of the amplification. Many different SBS media are available, such as N.sub.2, CH.sub.4 and SF.sub.6 gases, CCl.sub.4, TiCl.sub.4 and CS.sub.2 liquids, and solid glasses or crystals.
An input beam 8 is processed through a polarizer 10, beamsplitter 12 and quarter-wave plate 14 into the amplifier 4. After exiting the amplifier, the beam proceeds through an attenuating filter (or partially reflecting tilted mirror) 16 and is focused by a spherical lens 18 into the oscillator or seed generator 6, which functions as a self-pumped PCM. A weak phase conjugated seed is returned from the oscillator or generator 6 back to the amplifier 4, where it interacts with the relatively stronger input beam. Energy is transferred from the input to the phase conjugated return beam within the amplifier 4, and the amplified return beam is directed back through the quarter-wave plate 14. For monitoring purposes, the beamsplitter 12 together with mirrors 20, 22 and 24 are shown diverting a portion of the returned beam to a far-field camera 26, while the remainder of the return beam is directed by polarizer 10 onto a screen 28, upon which its near-field characteristics can be observed.
Systems of the type illustrated in FIG. 1 have been discussed in N. F. Andreev et al., "Nonstationary stimulated Mandel'shtam-Brillouin scattering of focused light beams under saturation conditions", Soviet Physics JETP, 58(4), October 1983, pages 688-692; V. N. Alekseev et al., "Investigation of wavefront reversal in a phosphate glass laser amplifier with a 12-cm output aperture", Soviet Journal of Quantum Electronics, 17(4), April 1987, pages 455-458; and A. F. Vasil'ev et al., "Stimulated Brillouin scattering at high values of the excess of the pump energy above the threshold", Soviet Journal of Quantum Electronics, 17(5), May 1987, pages 644-647. These publications deal with single input beams having spatially uniform intensity distributions over the beam cross-sections. A similar approach with multiple beams of equal intensity was treated in N. G. Basov et al., "control of the characteristics of reversing mirrors in the amplification regime", Soviet Journal of Quantum Electronics, 11(10), October 1981 , pages 1335-1337. All of these references used collimated beams within the SBS amplifier. In another reference a spatially uniform but phase-aberrated beam was focused inside an SBS amplifier with a spherical lens to obtain high SBS gain within the amplifier; A. F. Vasil'ev, et al., "Effective reflection of radiation with a large angular divergence from an SBS-PC mirror", Optics and Spectroscopy, 63(1), July 1987, pages 133-134.
Although the above publications deal with beams of uniform spatial intensity, in most applications a cross-section of the input beam will reveal spatial variations in its intensity. If the input beam to the SBS amplifier is spatially non-uniform, it has been found that the near field of the output beam is quite different from the input beam, and that the far field fidelity is substantially reduced. This has been determined to result from the fact that the gain, and hence the SBS reflectivity, at any particular point within the amplifier is proportional to the input optical intensity at that point. Thus, high intensity areas of the beam ("hot spots") will have a higher reflectivity, and low intensity areas ("cold spots") will be weakly reflected. As a result, the intensity contrast and the shape of the output beam will differ from the input. A substantial difference between the input and output beams will also reduce the far-field fidelity. In some cases the cold spots can be lost from the output beam entirely, even though they may contain a substantial portion of the input beam energy.
The intensity-dependent amplification problem extends beyond SBS amplifiers to other optical amplification techniques that exhibit an intensity dependent gain, such as Raman amplification and four-wave mixing. Cold spots in the input beam can be lost entirely from the output beam, or at least attenuated more than the hot spots. This applies to both single beams with spatial intensity variations, and to multiple parallel beams, which for purposes of the invention may be considered to be a single composite beam with a "hot spot" at each of the individual beam locations.